This invention relates to a cable construction for optical fiber communication cables.
In the past few years there has been a significant effort expended attempting to develop optical fiber cables capable of efficiently transmitting optically encoded information. This is largely due to the potential ability of these optical cables to carry large amounts of information through a fiber which is substantially smaller and lighter than its electrical equivalent. The performance of these optical cables are however greatly subject to the application of exterior mechanical stress. This stress not only has the potential for destroying the optical fibers, but it can also degrade the optical transmission characteristics of the fibers. It is well known that even slight bending of the entire fiber (i.e. macrobending) or annular perturbations along the surface of the fiber (i.e. microbending) can introduce sever losses into the transmission characteristics for the fiber. A large amount of effort has therefore been expended in minimizing the amount of stress which is applied to the individual optical fibers within a fiber cable in order to minimize these losses.
For example, U.S. Pat. No. 4,235,511 describes an optical fiber cable construction in which several structural compartments are formed within the cable to house the individual optical fibers. These compartments are substantially larger than the optical fibers placed therein to ensure a loosely fitting relationship between the fiber and the compartment. As stated in the abstract of the above-referenced patent, this "loose fitting of the optical fibers overcomes the increased transmission losses and changed transmission bandwidth caused by lateral or compression forces inherently applied to the optical fibers of conventional optical fiber cable constructions".
As an alternative to this compartment design, U.S. Pat. No. 4,009,932 describes an optical fiber cable construction in which the fiber is located within an array of 3 or more metal filaments which are spaced from the optical fiber and disposed in planes intersecting with the axis of the fiber at substantially equal angles to the other. The optical fiber and the filaments surrounding the optical fiber are then embedded in a synthetic thermoplastic resin material. As is stated in column 2 beginning at line 27 ". . . in practice, the symmetrical arrangement of the three metallic filaments at planes arranged at 120.degree. to each other or of the four metallic filaments in orthogonal planes, provides in any stress plane, at least one filament resistant to tension and two filaments resistant to compression, or two resistant to tension and one of compression, such filaments having physical characteristics which enable them to oppose the stresses and prevent deformation of the optical fiber."
Neither of these constructions are without shortcomings. Compartmentalized cable results in a bulky over-sized cable which is inherently inflexible and therefore does not lend itself to easy manipulation. For many applications of optical fiber cables, this limitation is severe. Housing the optical fiber in an encapsulated matrix of discrete strength members provides a cable that is more easily manipulated, but one in which further stresses can be introduced and focused by the mere presence of the discrete strength members themselves. As is apparent from the description of this structure quoted above, a strength member in compression may be directly opposite a strength member in tension. These competing forces in the plane between the two strength members can introduce additional stresses within the encapsulating matrix that are ultimately directed upon the optical fiber contained therein.